The invention relates to a dynamic synchronous transfer mode (DTM) network, and in particular to a method of operation thereof, and a node for use therein, which,in preferred embodiments, overcome the unfair operation of the network caused by the dual-bus topology and hence the inherently asymmetric nature of the network.
Dynamic synchronous transfer mode (DTM) is an attempt to build the next generation of networking technologies on fast circuit switching basis.
The operation of DTM is based on multirate and either unicast or broadcast channels. It is designed for a unidirectional medium with multiple access. The total medium capacity is shared by all connected nodes. Previous proposals and implementations are based on dual-bus topology. The architecture can be extended to include a large number of connected buses using switching nodes.
FIG. 1 shows the general structure of a dual-bus DTM network. The most important elements of the DTM network are the nodes and the hosts. Nodes are networking devices connected to the dual bus. Hosts are end-devices with a simple interface that connects them to a node. Host-host communication is based on the assistance of nodes. Nodes are responsible for resource allocation, connection establishment and release along the bus.
The total communication channel on the physical shared medium is realised by a time division multiplexing scheme. The total capacity of the bus is divided into cycles of 125 microseconds, which are further divided into slots. A slot consists of a 64-bit data-word and some additional management bits. The sequence of slots at the same position in successive cycles is called a DTM channel.
There are two types of slots (and thereby two DTM channels); data and static slots. Data slots are used for data transfer. The number of data channels specifies the bit-rate of a DTM connection. There is a token for each DTM channel, which is assigned to one of the nodes. Both free and used channels are assigned to nodes. Each channel has exactly one owner at a time. If a node has the right to use a channel, then it has full control of it; it can setup a connection on it, send data within the connection, release a connection using the channel, or give the channel ownership to another node.
According to the known DTM protocol, the sender node is responsible for channel reservation regardless of whether or not it is the initiator of a call. This is the most obvious solution if point-to-multipoint (multicast) connections are used.
However, consider the scenario where nodes generate point-to-point calls (as senders) with the same statistical characteristics. The destination of the calls (receiver) is chosen with uniform distribution, ie. there is an equal probability that a node sends data to any of the nodes on the bus.
In this scenario, though nodes generate the same amount of traffic to the dual-bus, the load distribution on the unidirectional buses will not be uniform.
If we consider only one of the unidirectional buses: a node at the end of the bus cannot transmit data, therefore, it has no need to reserve channels; a node at the beginning of the bus transmits all its traffic to the same bus, therefore, it needs to reserve twice the average number of channels. The blocking probability for a node, and the average setup time for the node will depend on the number of reservations made by the node. As a result, this known protocol leads to unfair treatment of the nodes.
According to a first aspect of the invention, there is provided a method of operation of a dynamic synchronous transfer mode network comprising a plurality of buses, wherein, when a first node wishes to initiate a call with a second node:
the first node selects a bus in the network for communication with the second node;
the first node calculates, for the selected bus, a ratio based on the number of previous reservations made by the first node and the number of previous reservations relating to calls to or from the first node; and
if the calculated ratio is below a threshold, the first node reserves one or more channels for the call; and
if the calculated ratio is above the threshold, the first node requests the second node to reserve one or more channels for the call.
This has the advantage that the calculated ratio can be controlled for each node, such that the ratios for each node in the network, and hence the blocking probabilities, and the average setup times, for the nodes, can be made similar.
Preferably, the network is a dual bus network, and each bus is unidirectional.
Preferably, the ratio calculated by the first node is the ratio of the number of channels previously reserved by the first node to the number of previous channels used for calls to or from the first node.
This has the advantage that it is the number of reserved channels which determines the blocking probability.
Preferably, the threshold for the calculated ratio is 50%.
This has the advantage that the ratio can in principle be controlled to 50% for each node, which means that the blocking probabilities, and the average setup times, for all nodes, can be made the same.
Preferably, when the first node requests the second node to reserve one or more channels for the call, it does so by sending a Query message to the second node.
This has the advantage that this message already exists in the existing DTM protocol.
According to a second aspect of the invention, there is provided a node for use in a dynamic synchronous transfer mode network, wherein the node comprises:
a selecting device for selecting a bus, when the node wishes to initiate a call with a second node, the selected bus being for communication with the second node;
a processor for calculating, for the selected bus, a ratio based on the number of previous reservations made by the node and the number of previous reservations relating to calls to or from the node; and
a channel reservation device, for, if the calculated ratio is below a threshold, reserving one or more channels for the call; and for, if the calculated ratio is above the threshold, requesting for the second node to reserve one or more channels for the call.
According to a third aspect of the invention, there is provided a dynamic synchronous transfer mode network, comprising a plurality of nodes and a plurality of buses joining the nodes, wherein each node comprises:
a selecting device for selecting a bus, when the node wishes to initiate a call with a second node, the selected bus being for communication with the second node;
a processor for calculating, for the selected bus, a ratio based on the number of previous reservations made by the node and the number of previous reservations relating to calls to or from the node; and
a channel reservation device, for, if the calculated ratio is below a threshold, reserving one or more channels for the call; and for, if the calculated ratio is above the threshold, requesting the second node to reserve one or more channels for the call.